1. Field of the Invention
The present invention relates to a receiving equipment using a code division multiple access (CDMA) system, particularly to a synchronous capture circuit for the CDMA receiving equipment used in a mobile communication system.
2. Description of the Prior Art
Generally, in CDMA system, a transmitting equipment transmits data signal by spectrum diffusion modulation using diffusion signature, and a receiving equipment demodulates the received data by the inverse diffusion using a replica of the diffusion signature, for example, M (Maximum Length Code) sequence signature or GOLD signature.
The above-mentioned receiving equipment in CDMA system is provided with a synchronous capture circuit which estimates correctly the phase of diffusion signature (diffusion signal generation timing) for inversely diffusing the received signal. Particularly, in the receiving equipment, the synchronous capture circuit estimates the diffusion signature generation timing within the accuracy of one period (one chip) of the diffusion signature generation timing in the transmitting equipment. Then, a diffusion signature generator of an inverse diffusion circuit in the receiving equipment starts acting at the above-mentioned generation timing.
Here, referring to FIG. 3, one of conventional CDMA synchronous capture circuit is explained. The synchronous capture circuit shown in FIG. 3 is provided with receiving antenna 11 for receiving the signal from the transmitting equipment (not shown), quasi synchronous detector 12 for converting the received signal to base band signal, A/D converter 13 for converting the base band signal to digital data, correlator 14 for calculating correlation value from the digital data after A/D conversion, phase shifter 15 for shifting the diffusion signature generation timing by prescribed time, memory 16 for storing the correlation value of one period of the diffusion signature, received signal level detector 17 for detecting the maximum correlation value in one period of the correlation value, and clock generator 18 for generating clock. Further, correlator 14 is provided with multiplier 141, integrator 142, and diffusion signature generator 143 for generating diffusion signature at prescribed timing.
In the CDMA synchronous capture circuit shown in FIG. 3, the RF signal received by receiving antenna 11 is converted to base band signal by quasi synchronous detector 12 and then, converted to digital signal by AID converter 13.
Then, the digital signal is fed to correlator 14.
Correlator 14 multiplies the diffusion signature sequence outputted from diffusion signature generator 143 and the digital signal outputted from D/C converter 13, in one chip unit. Then, the output of multiplier 141 is inputted into integrator 142, and accumulated over the length of the diffusion signature sequence.
The output from integrator 142 becomes the correlation value at a certain diffusion signature generation timing in the diffusion signature sequence.
The output of correlator 14 is stored in memory 16. Particularly, after the correlation values were outputtted from correlator 14, the phase of diffusion signature generator 143 is shifted by prescribed time smaller than the chip rate of the diffusion signature and similarly calculates the correlation value of the received signal using the diffusion signature generation timing shifted as mentioned above, and stores the calculation result in memory 16.
Thus, the correlation values over at least one period (one chip) of the diffusion signature sequence are calculated and stored in memory 16.
Next, signal level detector 17 selects the delay position of the received signal that has the maximum correlation value from the correlation value stored in memory 16. Using this delay position of the received signal, the received signal is inversely diffused by the inverse diffusion circuit (not shown) by using the diffusion signature sequence generated from the above-mentioned delay position of the received signal as a replica.
Besides, the amplitude and phase of the signal received by the receiving equipment varies at all times, because the transmission line between a base station and a mobile station in mobile communication. Accordingly, the quality of the maximum correlation value is improved, if the correlation value over one period (one chip) of the diffusion signature to be stored in memory 16 is calculated more than twice.
In mobile communication, because a communication equipment moves, the communication equipment receives multipath waves reflected by a plurality of obstacles as well as a direct wave from a transmitter. These multipath waves arrive shortly after the direct wave in urban area, because many obstacles such as buildings stand nearby, while they arrive after much delay in suburbs, because there are few obstacles near the mobile transmitter/receiver.
By using the CDMA system, the multipath can be separated, if the time interval of the generation of multipath waves is greater than one chip of the diffusion signature. Further, the quality of received signal can be improved by the path diversity by synthesizing multipath (RAKE synthesis).
However, in order to implement RAKE synthesis in the suburbs as well as in the urban area, it is necessary to detect multipath presicely. Particularly, in the suburbs, search range must be long enough to cover the arrival of the multipath waves. However, the longer the search range is, the more enormous is the quantity of processing for the calculation of the correlation values in the synchronous capture circuit. Further, the reduction of the processing time results in the increase in the number of the correlator, the increase in the enlargement of the whole circuit, and the increase in power consumption with the broadening of the frequency range.
Thus, conventional CDMA synchronous capture circuit has a disadvantage that the wider the synchronous capture range is, the longer the processing time is.
Therefore, an object of the present invention is to provide a CDMA synchronous capture circuit for reducing the quantity of calculation of the correlation values.
The CDMA synchronous capture circuit of the present invention calculates correlation value by using a part of received data for the detection of the peak position by using the correlation values and then, decides an upper temporal peak position where the correlation values are great.
Then, the CDMA synchronous capture circuit of the present invention calculates the remaining correlation values, giving priority over the temporal peak position, by using the remaining received data, and finally decides a peak value from the correlation level by adding the above-mentioned two kinds of correlation values. Whereby the quantity of processing is reduced.
The CDMA synchronous capture circuit of the present invention calculates temporal correlation values by using the the first xe2x80x9ckxe2x80x9d calculations among xe2x80x9cnxe2x80x9d (n:integer. k less than n) calculations of correlation value for accumulating diffusion signature sequences in a search range, and detects the upper xe2x80x9cmxe2x80x9d phase positions, and holds the received phases and the correlation values only for such signature generation timings that each correlation value calculated by the first xe2x80x9ckxe2x80x9d calculations is greater than a prescribed threshold value. At first, the upper xe2x80x9cmxe2x80x9d phase positions are added to the remaining xe2x80x9c(nxe2x88x92k)xe2x80x9d correlation values to obtain the final correlation value.
Next, remaining xe2x80x9c(nxe2x88x92k)xe2x80x9d correlation values are added to the correlation values corresponding to the remaining received phases. Then, if the result of addition is greater than the correlation values obtained beforehand for the xe2x80x9cmxe2x80x9d phase positions, the values obtained beforehand are replaced by the greater values.
As explained above, the CDMA synchronous capture circuit for detecting the peak position in the prescribed search range decides at first a temporal peak position by the correlation calculation by using a part of the received data and next, calculates the final peak value by calculating the remaining correlation values, by using the remaining received data and by adding two correlation values. On the other hand, the CDMA synchronous capture circuit of the present invention does not calculate any correlation value at the diffusion signature generation timing, where the accumulated correlation values by the data received beforehand are smaller than a prescribed threshold value.
Accordingly, the processing time for detecting the maximum peak can effectively be shortened, when the search range is very broad.
Further, according to the present invention, the quantity of the calculation of correlation values per one search can effectively be reduced. Thus, the number of the correlators can effectively be reduced, and the scale of the whole circuit can effectively be reduced.